The invention relates to fan blades and in particular to fan blades reinforced with a high elongation fiber composite.
Fan blades used in jet engine applications are susceptible to foreign object impact damage such as bird ingestion events. Blades made of graphite fiber reinforced composite material are attractive due to their high overall specific strength and stiffness. However, graphite composites are particularly prone to brittle fracture and delamination during foreign object impacts due to their low ductility. Blade leading edges, trailing edges, and tips are particularly sensitive because of the generally lower thickness in these areas and the well-known susceptibility of laminated composites to free edge delamination. In addition blade geometry and high rotational speeds relative to aircraft speeds cause ingested objects to strike the blade near the leading edge. The material near the suction and pressure surfaces of the composite are most prone to fracture due to the local bending deformations typically associated with such events.
Metallic guards bonded to the composite blade are known to provide impact damage protection. However, the high density of these materials limit their use. In addition, blades can be ruggedized by increasing the airfoil thickness either locally or over a wide area. Blade thickening results in an aerodynamic penalty as well as a weight penalty.